The present invention relates to a micromachine switch used in a milliwave band to microwave band.
Switch devices such as a PIN diode switch, HEMT switch, micromachine switch, and the like are used in a milliwave band to microwave band. Of these switches, the micromachine switch is characterized in that the loss is smaller than that of the other devices, and a compact high-integrated switch can be easily realized.
FIG. 13 is a perspective view showing the structure of a conventional micromachine switch. FIG. 14 is a plan view of the micromachine switch shown in FIG. 13.
A micromachine switch 101 is constructed by a switch movable element 111, support means 112, and switch electrode 113. The micromachine switch 101 is formed on a dielectric substrate 103 together with two RF microstrip lines 102a and 102b. A GND plate 104 is disposed on the lower surface of the dielectric substrate 103.
The microstrip lines 102a and 102b are closely disposed apart from each other at a gap G. The switch electrode 113 is disposed between the microstrip lines 102a and 102b on the dielectric substrate 103. The switch electrode 113 is formed to have a height lower than that of each of the microstrip lines 102a and 102b. 
The switch movable element 111 is arranged above the switch electrode 113. A capacitor structure is formed by the switch electrode 113 and switch movable element 111.
As shown in FIG. 14, since a length L of the switch movable element 111 is larger than the gap G, two ends of the switch movable element 111 oppose the end portions of the microstrip lines 102a and 102b, respectively. The switch movable element 111 is formed to have a width equal to the width W of each of the microstrip lines 102a and 102b. 
The switch movable element 111 is cantilevered on the support means 112 fixed on the dielectric substrate 103.
As shown in FIG. 13, the switch movable element 111 is generally arranged above the microstrip lines 102a and 102b. With this structure, since the switch movable element 111 is not in contact with the microstrip lines 102a and 102b, the micromachine switch 101 is in an OFF state. At this time, a little high-frequency energy is transmitted from the microstrip line 102a to the microstrip line 102b. 
When, however, a control voltage is applied to he switch electrode 113, the switch movable element 111 is pulled down by an electrostatic force. When the switch movable element 111 is brought into contact with the microstrip lines 102a and 102b, the switch movable element 111 is set in an ON state. At this time, the high-frequency energy from the microstrip line 102a is transmitted to the microstrip line 102b through the switch movable element 111.
As described above, the two ends of the switch movable element 111 oppose the microstrip lines 102a and 102b, respectively. Accordingly, the capacitor structures are also formed between the switch movable element 111 and the microstrip lines 102a and 102b. 
This makes the capacitive coupling between the switch movable element 111 and microstrip lines 102a and 102b so that the high-frequency energy from the microstrip line 102a leaks out into the microstrip line 102b even if the micromachine switch 101 is in the OFF state. That is, in the conventional micromachine switch 101, an OFF isolation characteristic is poor.
In the microwave switching circuit, for example, the isolation of approximately 15 dB or more is required.
The present invention has been made to solve the above problem, and has as its object to improve the OFF isolation characteristic of the micromachine switch.
In order to achieve the above object, the present invention comprises at least two distributed constant lines disposed close to each other, a movable element arranged above the distributed constant lines so as to oppose the distributed constant lines and connecting the distributed constant lines to each other in a high-frequency manner upon contacting the distributed constant lines, and driving means for displacing the movable element by an electrostatic force to bring the movable element into contact with the distributed constant lines, wherein the movable element includes a projection formed by notching at least one end of an edge of the movable element which is located on at least one distributed constant line side, and a width of the projection serving as a length in a direction parallel to the widthwise direction of the distributed constant lines is smaller than a width of each of the distributed constant lines. That is, at least one end of the movable element is notched to form the projection having the width (the length in the direction parallel to the widthwise direction of the distributed constant lines) smaller than that of the distributed constant line, and the projection is made to oppose the distributed constant line. This decreases the opposing area between the movable element and the distributed constant line, thereby reducing the capacitive coupling of them. Therefore, the OFF isolation characteristic of the micromachine switch can be improved. In addition, since the width of the movable element on the gap between the distributed constant lines becomes larger as compared to the case in which a movable element having the rectangular shape and the width smaller than that of the distributed constant line is used, the present invention can obtain ON reflection characteristic better than that in the above case.
In the present invention, at least one distributed constant line opposing the projection of the movable element does not oppose a movable element main body serving as a portion of the movable element expect for the projection. That is, only the projection of the movable element opposes the distributed constant line. Accordingly, the width of the movable element opposing the distributed constant line is smaller than that of the distributed constant line as a whole. Thus, an OFF isolation characteristic similar to that in the case in which the movable element having the rectangular shape and the width smaller than that of the distributed constant line is used can be realized, and an ON reflection characteristic better than that in that case can be obtained.
In the present invention, at least one distributed constant line opposing the projection of the movable element also opposes a part of a movable element main body serving as a portion of said movable element expect for the projection. That is, the projection of the movable element and the part of the movable element main body oppose the distributed constant line. Thus, the opposing area between the movable element and the distributed constant line is increased as compared to the above invention, and, an OFF isolation characteristic can be improved as compared to the prior art.
In this case, the movable element main body of the movable element is formed to have a width equal to the width of the distributed constant line. Thus, there is almost no discontinuous portion between the distributed constant line and movable element, and an ON reflection characteristic better than that in the above invention can be obtained.
In the present invention, the projection of the movable element has a rectangular shape. When the rectangular projection is formed by notching two ends of the movable element, the opposing area between the movable element and the distributed constant line is a predetermined area even if the positioning error occurs in the longitudinal direction of the movable element.
In the present invention, the width of the projection of the movable element near the movable element main body serving as a portion of the movable element expect for the projection is made larger than that away from the movable element main body.
Since the width of the projection of the movable element near the movable element main body serving as a portion of the movable element expect for the projection is made larger than that away from the movable element main body, the strength of the projection increases.
Also, the present invention comprises at least two distributed constant lines disposed close to each other, a movable element arranged above the distributed constant lines so as to oppose the distributed constant lines and connecting the distributed constant lines to each other in a high-frequency manner upon contacting the distributed constant lines, and driving means for displacing the movable element by an electrostatic force to bring the movable element into contact with the distributed constant lines, wherein at least one distributed constant line includes a projection formed by notching at least one end of an edge of at least one distributed constant line on the movable element side, and a width of the projection is smaller than a length, serving as a width of the movable element, in a direction parallel to the widthwise direction of the distributed constant lines. That is, at least one end of the distributed constant line is notched to form the projection having the width (the length in the direction parallel to the widthwise direction of the distributed constant lines) smaller than that of the movable element, and the projection is made to oppose the movable element. This decreases the opposing area between the movable element and the distributed constant line, thereby reducing the capacitive coupling of them. Therefore, the OFF isolation characteristic of the micromachine switch can be improved. In addition, a good ON reflection characteristic can be obtained as compared to the case in which a movable element having the rectangular shape and the width smaller than that of the distributed constant line is used.
In the present invention, the movable element does not oppose a distributed constant line main body serving as a portion, expect for the projection, of at least one distributed constant line having the projection. That is, only the projection of the distributed constant line opposes the movable element. Accordingly, an OFF isolation characteristic similar to that in the case in which the movable element having the rectangular shape and the width smaller than that of the distributed constant line is used can be realized, and an ON reflection characteristic better than that in that case can be obtained.
In the present invention, the movable element also opposes a part of a distributed constant line main body serving as a portion, expect for the projection, of at least one distributed constant line having the projection. That is, the projection of the distributed constant line and the part of the distributed constant line main body oppose the movable element. Thus, an OFF isolation characteristic can be improved as compared to the above invention.
In this case, the movable element may be formed to have a width equal to the width of each of the distributed constant line main bodies. Thus, an ON reflection characteristic better than that in the above invention can be obtained.
In the present invention, the projection of at least one distributed constant line has a rectangular shape. Thus, even if the positioning error occurs in the longitudinal direction of the movable element, the opposing area between the movable element and the distributed constant line is a predetermined area.
In addition, the present invention comprises at least two distributed constant lines disposed close to each other, a movable element arranged above the distributed constant lines so as to oppose the distributed constant lines and connecting the distributed constant lines to each other in a high-frequency manner upon contacting the distributed constant lines, and driving means for displacing the movable element by an electrostatic force to bring the movable element into contact with the distributed constant lines, wherein at least one distributed constant line includes a first projection formed by notching at least one end of an edge of at least one distributed constant line on the movable element side, and the movable element includes a second projection so formed as to oppose the first projection of at least one distributed constant line by notching at least one end of an edge of the movable element. With this structure, an OFF isolation characteristic of the micromachine switch can be improved. In addition, a good ON reflection characteristic can be obtained as compared to the case in which a movable element having the rectangular shape and the width smaller than that of the distributed constant line is used.
In the present invention, at least an entire lower surface of the movable element is made of a conductor.
In the present invention, the movable element is made of a conductive member and an insulating thin film formed on an entire lower surface of the conductive member.
In the present invention, the driving means comprises an electrode which is disposed apart between the distributed constant lines so as to oppose the movable element and to which a driving voltage is selectively applied.
In the present invention, the invention further comprises support means for supporting the movable element, the driving means is made of an upper electrode attached to the support means and a lower electrode disposed under the upper electrode and opposing the upper electrode, and a driving voltage is selectively applied to at least one of the upper and lower electrodes.